A. Field of the Invention
The present invention relates to an organic EL device, in particular, to an organic EL device using a resin substrate or a flexible film substrate.
B. Description of the Related Art
Since the publication of an organic EL device having a double-layered structure and exhibiting high efficiency by C. W. Tang et al. of Eastman Kodak Company in 1987, various types of organic EL devices have been developed, and practical application has begun in some of the devices. (See C. W. Tang et al., Appl. Phys. Lett. 51, 913 (1987).) In this circumstance, the developments for practical application are actively and rapidly being made on full-color organic EL displays using an organic EL device as a next generation display.
Critical issues in practical application of color displays are capability of precise color display and long term stability, including color reproducibility. A color organic EL display, however, has a drawback that the light emission characteristic (current-brightness characteristic) significantly degrades after a certain period of operation.
One of the most important causes of this degradation of light emission characteristic is the growth of dark spots. The dark spot is a spot of failure of light emission. The dark spots grow by oxidation during operation or storage and expand to the entire surface of light emission. The dark spots are considered to occur due to oxidation or aggregation in the laminated material composing the device by oxygen or moisture in the device. The growth progresses during storage as well as in operation. The growth is: (1) accelerated by oxygen or moisture around the device, (2) affected by oxygen or moisture existing as an adherent to the organic laminated films, and (3) affected by moisture adhering to the parts for fabricating the device or moisture invaded in the manufacturing process.
To prevent moisture from invading an organic EL device, it has been proposed to provide an insulating barrier layer against the moisture and oxygen on the substrate. Known techniques for the barrier layer against moisture and oxygen include provision of an organic resin such as polyimide-modified silicone resin (see, for example, Japanese Unexamined Patent Application Publication Nos. H5-134112, H7-218717, and H7-306311) and provision of an inorganic oxide layer having a thickness in the range of 0.01 to 200 μm (see for example, Japanese Unexamined Patent Application Publication No. H8-279394). Such an inorganic oxide layer must be highly moisture proof in order to maintain a long life of the organic light emitting layer.
A method of fabricating a color filter provided with one or more color filter layers on a substrate is known, in which SiOx or SiNx is formed by DC sputtering on a polymer film formed on the color filter layer, and the method has an effect to improve adhesiveness of a transparent conductive film. (See Japanese Unexamined Patent Application Publication Nos. H7-146480 and H10-10518.) A method to sinter low melting point glass also is known. (See Japanese Unexamined Patent Application Publication No. 2000-214318.)
Active studies have been made regarding the performance degradation of organic EL devices, and various causes therefor have been reported. Among the causes of degradation, one of the serious problems is performance degradation by the heat generated in the device operation, since the organic EL layer including a light emitting layer is composed of a poor heat resistive material.
The inorganic oxide film such as SiOx or SiNx is known to be liable to generate pinholes through the film. Such pinholes allow moisture or oxygen to reach the organic EL layer through the pinholes and the dark spots may grow.
When a substrate covered with polymer or a resin substrate is used, the polymer or the resin is known to contain moisture or oxygen. The moisture or the oxygen is known to be released from the substrate by heat generated by operation of the organic EL device, and reaches the organic EL layer to promote the growth of dark spots. In addition, such substrates include significant size irregularities on their surface, and some of these irregularities may not be thoroughly covered with the barrier layer against moisture and oxygen. If an exposed portion exists, the moisture or oxygen in the substrate leaks through the exposed portion and causes the dark spots to grow. In the case of a barrier layer against moisture and oxygen made of an organic resin, in particular, once a dark spot is formed, the growth is noticeable.
Consequently, a structure is eagerly demanded that surely obstructs invasion of moisture and oxygen into an organic EL layer and prevents occurrence of dark spots even in the cases of a substrate covered with polymer and a resin substrate.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.